A great challenge facing neuroscience is to understand how genes, molecules, cells, circuits, and systems interact to generate social behavior. Fruit flies ( Drosophila melanogaster ) offer a powerful model system to address questions of this magnitude. These animals display genetically specified, sexually dimorphic patterns of fighting behavior via sex-specific splicing of the fruitless gene. Here, we show that sexually dimorphic behavioral patterns displayed during aggression are controlled by specific subgroups of neurons expressing male forms of fruitless proteins (Fru M ). Using the GAL4/UAS system to manipulate transformer expression, we feminized or masculinized different populations of neurons in fly nervous systems. With a panneuronal elav-GAL4 driver, male patterns of fighting behavior were transferred into females and female patterns into males. We screened 60 Gal4 lines that express the yeast transcription factor in different patterns in fly central nervous systems and found five that showed abnormal same-sex courtship behavior. The sexually dimorphic fighting patterns, however, were completely switched only in one and partially switched in a second of these lines. In the other three lines, female patterns of aggression were seen despite a switch in courtship preference. A tight correspondence was seen between Fru M expression and how flies fight in several subgroups of neurons usually expressing these proteins: Expression is absent when flies fight like females and present when flies fight like males, thereby beginning a separation between courtship and aggression among these neurons.